The present invention relates to a fuel cell system that generates electricity by reforming raw fuel gas which is vaporized raw fuel liquid and feeding it to a fuel cell, as well as to a fuel evaporator that can be suitably applied to the fuel cell system.
Conventional fuel cell systems are known that inject raw fuel liquid comprising a mixture of methanol and water or the like into a fuel evaporator (evaporation chamber) via a raw fuel injection apparatus and vaporize the raw fuel liquid into raw fuel gas, and then reform the raw fuel gas with a reformer while removing the carbon monoxide to yield hydrogen-rich raw fuel gas and feeding the raw fuel gas to a fuel cell to generate electricity. However, when a fuel cell system having such a construction is used under conditions with extreme load fluctuation, such as when it is mounted for use in a fuel cell-powered electric automobile, sudden injection of the raw fuel liquid into the fuel evaporator based on the requirement for increased output makes it impossible to vaporize all of the raw fuel liquid, and this often creates liquid pools of raw fuel liquid (hereunder referred to as xe2x80x9cliquid poolsxe2x80x9d) in the fuel evaporator. When the fuel evaporator has not been adequately warmed up upon starting the fuel cell system, liquid pools tend to form due to a lack of heat for vaporization.
When liquid pools form in the fuel evaporator, the liquid pools vaporize due to heat retained inside even after terminating injection of the raw fuel liquid, and this produces raw fuel gas resulting in undesirable deterioration in the fuel evaporator response.
When the raw fuel liquid is a mixture, the highly volatile components of the formed liquid pools vaporize first thus creating an imbalance in the composition of the raw fuel gas, and the reformer often fails to exhibit adequate performance or the carbon monoxide cannot be sufficiently removed, thus lowering the performance of the fuel cell. In addition, it becomes impossible to satisfactorily control the humidity in the fuel cell, such that the fuel cell often fails to generate the prescribed output.
For these reasons, the present applicant has proposed in Japanese Patent Application No. Hei-11-125366 (unpublished) the fuel evaporator 100 shown in FIG. 23, with the object of effectively preventing creation of liquid pools for improved response of the fuel evaporator and to allow more rapid warming up of the fuel evaporator. The fuel evaporator 100 is provided with an evaporator body 110, a superheater 130 downstream from the evaporator body 110 and a raw fuel gas injection apparatus 140 above the evaporator body 110.
In the fuel evaporator 100, heated gas HG serving as a high temperature heat medium obtained by catalytic combustion of off gas (hydrogen-containing gas) generated in a fuel cell (not shown) at a catalytic combustor (not shown) is supplied as a heat source. The heated gas HG passes from an inlet 112in through a plurality of U-shaped heat medium tubes 112 arranged in the evaporation chamber 111 of the evaporator body 110, and reaches an outlet 112out. The heated gas HG then passes through a heated gas conduit 113 provided under the evaporation body 110, and is conducted to a superheater 130 mounted downstream from the evaporator body 110. Raw fuel liquid FL composed of a methanol and water mixture is injected as a mist from a fuel injection apparatus 140 and is heated to vaporization with the heat medium tubes 112 into raw fuel gas FG. The raw fuel gas FG is superheated as it passes through vapor tubes 131 of the superheater 130 and is conducted to a reformer (not shown) downstream from the superheater 130. The heat medium tubes 112 are U-shaped with the top and bottom tubing as the horizontal tubing on either side of the curved sections Rxe2x80x2, and as shown in FIG. 24, they are evenly arranged from top to bottom (vertically) and from side to side (horizontally). FIG. 24 is a cross-sectional view of FIG. 23 along line Dxe2x80x94Dxe2x80x2.
In this fuel evaporator 100, the bottom 111b of the evaporation chamber 111 in the evaporator body 110 also serves as the top 113t of the heated gas conduit 113. Thus, since heat is also supplied from the bottom 111b of the evaporation chamber 111, creation of liquid pools is prevented, and even when liquid pools are created they are rapidly vaporized. The response of the fuel evaporator 100 is therefore improved.
The fuel evaporator must accomplish rapid and efficient vaporization of raw fuel liquid upon starting up or under extreme load fluctuations in order to yield the raw fuel gas.
In a conventional fuel evaporator 100, however, the amount of heat on the bottom 111b or sides is not so great, and therefore liquid pools have formed on the bottom 111b and sides of the evaporation chamber 103 of the fuel evaporator 100, due to pooling of raw fuel liquid that fails to vaporize.
Furthermore, since the heat medium tubes 112 are arranged horizontally, the raw fuel liquid FL injected from the raw fuel injection apparatus 140 has often pooled on the surface of the heat medium tubes 112, thus reducing the heat transfer efficiency of the heat medium tubes 112. The reduced heat transfer efficiency of the heat medium tubes 112 increases the amount of raw fuel liquid FL that falls under the evaporation chamber 111 without vaporizing above the evaporation chamber 111, but since the heat medium tubes 112 are evenly arranged from top to bottom and from side to side, there have been heat medium tubes 112 that do not contact with the falling raw fuel liquid FL; i.e., that contribute little to vaporization of the raw fuel liquid FL.
In addition, since the temperature of the heated gas flowing through the heat medium tubes 112 is lower at the lower side of the heat medium tubes 112 (at the heat medium tube outlets 112out), the amount of heat is insufficient at the bottom of the evaporation chamber 111 so that the raw fuel liquid FL often cannot vaporize at the lower side of the heat medium tubes 112, tending to result in formation of liquid pools.
When pooled raw fuel liquid is present in the evaporation chamber 111, the pooled raw fuel liquid can flow into the superheating chamber in cases where it runs along an upgrade. The unvaporized raw fuel liquid that has entered the superheating chamber can reach the reformer through the vapor tubes of the superheating chamber, and unreacted raw fuel liquid flowing into the reformer is a cause of reduced reforming efficiency and deterioration of the reformer.
Consequently, it is an object of the present invention to provide a fuel evaporator whereby warming up of the fuel evaporator can be rapidly accomplish liquid pools in the evaporator can be prevented with a simple structure, by effective utilization of the heat from the catalytic combustor.
It is another object to provide a fuel evaporator provided with a piping structure that allows rapid vaporization of raw fuel and effective utilization of the potential heat of the high temperature heat medium for vaporization of raw fuel liquid.
It is yet another object to provide a fuel evaporator that can effectively prevent flow of unvaporized raw fuel liquid in the fuel evaporator out of the evaporation chamber, and that can also prevent flow of unvaporized raw fuel liquid through the superheating chamber to the reformer.
It is yet another object to provide a fuel cell system with satisfactory response to loads by improved evaporation efficiency of raw fuel liquid in the fuel evaporator.
The present invention which overcomes the aforementioned problems is a fuel evaporator with an evaporation chamber that vaporizes raw fuel liquid with a high temperature heat medium, comprising a catalytic combustor installed adjacent to the evaporation chamber.
With this construction it is possible to apply more heat more rapidly to raw fuel liquid adhering as droplets onto the wall of the evaporation chamber or raw fuel liquid present as liquid pools, than by providing a separate combustor as in fuel evaporators of the prior art.
In the fuel evaporator according to the present invention, the catalytic combustor is preferably installed in close contact with the evaporation chamber.
With this construction it is possible to apply more heat more rapidly to raw fuel liquid adhering as droplets at sections in close contact with the catalytic combustor, or to raw fuel liquid pools.
In the fuel evaporator according to the present invention, preference is given to having a construction such that the contact side on which the catalytic combustor is in contact with the evaporation chamber forms the bottom surface of the evaporation chamber, and the bottom has a shape that follows the outer shape of the heat medium tube located nearest the bottom among the heat medium tubes in the evaporation chamber through which the high temperature medium flows.
With this construction it is possible to reduce the space for liquid pools under the evaporation chamber.
The fuel evaporator according to the present invention, preference is ginvn to having a construction such that the bottom of the catalytic combustor has a shape that is depressed from the periphery toward the center.
With this construction it is possible to increase the amount of heat near the center of the catalytic combustor compared to its outer periphery, for vaporization of more stored liquid.
The fuel evaporator according to the present invention may have an evaporation chamber that vaporizes raw fuel liquid with a high temperature heat medium and a high temperature heat medium conduit through which the high temperature heat medium flows after the raw fuel liquid has been vaporized. It is also preferable that a catalytic combustor is provided adjacent to the evaporation chamber, and the high temperature heat medium conduit is installed at a location of the evaporation chamber other than the location adjacent to the catalytic combustor.
With this construction it is possible to promote vaporization of raw fuel liquid with heat from the catalytic combustor that has not been utilized in the prior art. Rapid warming up can also be accomplished. Since the evaporation chamber can also be heated and warmed from the sides, it is possible to rapidly vaporize splashing droplets of raw fuel liquid adhering to the sides of the evaporation chamber. Rapid warming can also be accomplished.
Incidentally, the term xe2x80x9cadjacentxe2x80x9d in the claims includes cases where the catalytic combustor is provided in close contact with the evaporation chamber, as in this mode of the invention. When the evaporation chamber and the catalytic combustor are in close contact, the heat generated from the catalytic combustor is transferred to the evaporation chamber by conducted heat transfer. On the other hand, when a gap (space) is present between the evaporation chamber and the catalytic combustor, the heat generated by the catalytic combustor is transferred to the evaporation chamber by radiant heat transfer and convection heat transfer. That is, xe2x80x9cadjacentxe2x80x9d means that the heat generated by the catalytic combustor is transferred to the evaporation chamber by at least one heat transfer means from among conduction, radiation and convection, and this promotes vaporization of the raw fuel liquid (prevents formation of liquid pools) in the evaporation chamber.
The fuel evaporator according to another aspect of the present invention is provided with an evaporation chamber that vaporizes raw fuel liquid by heat received from a plurality of heat medium tubes through which a high temperature heat medium flows to produce raw fuel gas. A raw fuel injection portion that injects said raw fuel liquid into the heat medium tubes is provided in the evaporation chamber, and the heat medium tubes are situated in such a manner that those nearer the raw fuel injection portion are sparse and those further are dense.
Because the heat medium tubes are densely arranged at the section distant from the raw fuel injection portion in this construction, a high temperature zone is formed at the section distant from the raw fuel injector. The path of the injected raw fuel liquid is maintained in the proximity of the raw fuel injector, and therefore the raw fuel liquid spreads throughout the evaporation chamber. The fuel evaporator of the invention therefore exhibits highly satisfactory vaporization efficiency and does not form liquid pools easily.
The heat medium tubes according to the invention may be conduits of any type or shape, such as straight pipes or U-shaped or S-shaped pipes.
In this construction, it is possible to avoid noise due to oscillation or damage to the apparatus by interweaving wide and narrow sized conduits, by interweaving long and short conduits, by interweaving sparse and dense conduit arrangements or by appropriately combining these.
The fuel evaporator according to another aspect of the present invention may have a construction such that said evaporation chamber is provided with a tube holder portion that holds the heat medium tubes which are adjacent to the catalytic combustor and through which said high temperature heat medium flows, and has a section heated from said catalytic combustor, wherein a slanted section slanted from the horizontal with said tube holder portion directed downward is provided for at least part of said heat medium tubes.
With this construction, droplets of the raw fuel liquid injected on the outer surface of the heat medium tubes are moved in a fixed direction on the slanted section, to prevent significant growth with horizontal movement on the heat medium tubes. In addition, since the tube holder portion provided just directly above the catalyst layer outlet of the catalytic combustor and having a heated section is at high temperature, the droplets that have traversed the slanted section of the heat medium tubes up to the tube holder portion can be rapidly vaporized as they fall down on the evaporation chamber bottom while being heated by the high temperature tube holder.
The fuel evaporator according to another aspect of the present invention may have a construction such that the heat medium tubes are provided with a turbulence generating portion that disturbs the flow of the high temperature heat medium.
By thus providing a turbulence generating portion in the heat medium tubes, the flow is converted from a laminar flow to a turbulent flow as the high temperature heat medium passes through the heat medium tubes, thus creating an even temperature distribution in the radial direction of the tube. As a result, since a sufficient amount of heat can be applied to the outer surface of the tubes than with a laminar flow, the potential heat of the high temperature heat medium can be more effectively utilized for vaporization of the raw fuel liquid.
The fuel evaporator according to another aspect of the present invention may also have an evaporation chamber that vaporizes raw fuel liquid into raw fuel gas with a high temperature heat medium, the fuel evaporator comprising an evaporation chamber outlet that allows the raw fuel gas to flow out of the evaporation chamber formed in the evaporation chamber, and a raw fuel liquid shield that prevents flow of the raw fuel liquid provided under the evaporation chamber outlet.
With this construction, a raw fuel liquid shield that prevents outflow of the raw fuel liquid is provided under the evaporation chamber outlet. Thus, even when the unvaporized raw fuel liquid pools in the evaporation chamber it is possible to prevent the pooled raw fuel liquid from flowing out into the superheating chamber.
The fuel evaporator according to another aspect of the present invention may have a construction such that a raw fuel liquid storage section which stores the raw fuel liquid that has pooled under said evaporation chamber is formed in the evaporation chamber, and the raw fuel liquid shield extends from the lower end of the evaporation chamber outlet to at least a point above the raw fuel storage section.
With this construction, the raw fuel liquid shield extends from the lower end of the evaporation chamber outlet to a point above the raw fuel liquid storage section. Consequently, even when the raw fuel liquid that has pooled in the raw fuel liquid storage section pools up to a height under the evaporation chamber outlet, for example, it is possible to prevent the raw fuel liquid from flowing out from the evaporation chamber outlet.
The fuel evaporator according to another aspect of the present invention may have a construction such that ventilation means that prevents passage of liquid droplets of the raw fuel liquid while discharging the raw fuel gas is formed in the evaporation chamber outlet.
With this construction it is possible to prevent escape of raw fuel liquid droplets attempting to pass through the evaporation chamber outlet, while allowing outflow of the vaporized raw fuel gas. The ventilation means of the invention will typically be a metal mesh, punching plate or louver.
The fuel evaporator according to another aspect of the present invention may have a construction such that the evaporation chamber outlet conducts to a superheating portion which superheats raw fuel liquid that has been vaporized in the evaporation chamber by the high temperature heat medium that has passed through a high temperature heat medium conduit that conducts the high temperature heat medium that has vaporized the raw fuel liquid, vapor tubes that conduct raw fuel gas that has been discharged from the evaporation chamber outlet are installed in the superheater, and the raw fuel gas inlet for the vapor tubes is situated at a location lower than the raw fuel gas outlet.
With this construction, a raw fuel gas inlet for the vapor tubes provided in the superheating portion is situated at a location lower than the raw fuel gas outlet. Consequently, even when the unvaporized raw fuel liquid flows into the vapor tubes, it returns to the raw fuel gas inlet by the action of gravity, so that the raw fuel liquid flows out from the raw fuel gas inlet. It is therefore possible to effectively prevent the unvaporized raw fuel liquid from flowing out into the reformer.
Still another aspect of the present invention is a fuel cell system provided with a fuel evaporator having an evaporation chamber that vaporizes raw fuel liquid with a high temperature heat medium and an adjacent catalytic combustor that supplies the high temperature heat medium thereto, while also comprising a reformer that reforms raw fuel gas formed by vaporization of the raw fuel liquid into raw fuel gas, a fuel cell, and a raw fuel liquid tank that supplies the raw fuel liquid to the fuel evaporator.
With this construction, adhesion and pooling of raw fuel liquid in the evaporation chamber of the fuel evaporator can be effectively prevented, thus allowing the raw fuel gas to be appropriately supplied to the fuel cell. The fuel cell system thus exhibits satisfactory response. Moreover, since the catalytic combustor is provided adjacent thereto, the entire system can-be designed in a more compact form.
In the fuel cell system of the present invention, the fuel evaporator is preferably provided with a plurality of heat medium tubes through which the high temperature heat medium passes and a raw fuel injection portion that injects the raw fuel liquid into the heat medium tubes, wherein the heat medium tubes are situated in such a manner that those nearer the raw fuel injection portion are sparse and those further are dense.
With this construction, the raw fuel liquid can be spread more widely in the evaporation chamber of the fuel evaporator while the evaporation efficiency can be increased by the high temperature zone formed at a section distant from the raw fuel injector. It is therefore possible to achieve more suitable supply of the raw fuel gas to the fuel cell.